Transparent glass microspheres (i.e., beads) used in reflectors such as reflective sheets and road surface reflectors can be produced by, for example, melting methods. Such melting methods typically include melting a raw material composition in the form of particulate material. The liquid can then be quenched in water, dried, and crushed to form particles of a size desired for the final beads. The crushed particles are then passed through a flame having a temperature sufficient to melt and spheroidize the crushed particles. For most glasses this is a temperature of about 1000.degree. C. to about 1450.degree. C. Alternatively, the liquid can be poured into a jet of high velocity air. Beads are formed directly in the resulting stream. The velocity of the air is adjusted to control the size of the beads. These beads are normally composed of a vitreous material that is completely amorphous (i.e., noncrystalline), and hence, the beads are often referred to as "vitreous," "amorphous," or simply "glass" beads or microspheres.
Silica is a common component of glass-forming compositions. Alumina and zirconia have also been used in transparent glass beads to improve mechanical properties such as toughness, hardness, and strength. However, the amount of alumina and zirconia such beads can contain tends to be limited so as to avoid problems arising from crystallization, such as loss of transparency and processing difficulties. The most highly durable compositions comprise primarily alumina, zirconia, and silica with small amounts of modifiers (e.g., alkaline earth oxides). These compositions tend to have very high melting points and require special processing equipment such as a hydrogen flame or plasma torch. Lower melting compositions with high modifier content typically have poorer mechanical properties and provide only modest improvements over conventional glass beads. In addition, compositions with high modifier content (e.g., greater than about 25% by weight) can exhibit poor chemical durability. High durability fused beads described to date generally do not have a desirable refractive index unless the zirconia content is high (e.g., greater than about 55% by weight); however, high zirconia content tends to raise the melting point and the tendency to crystallize, creating difficulty in obtaining transparent microspheres without the use of a plasma gun or other specialized equipment.
Conventional barium titanate-based glass bead compositions are typically based on the compound BaTiO.sub.3, or the BaO/TiO.sub.2 eutectic, and can contain silica or a higher index oxide such as tin oxide or zinc oxide. They can contain greater than about 55% titania, although lower titania compositions comprising primarily lead oxide are also known. Barium titanate-based glass bead compositions are typically low durability glass beads, contain greater than 20% alkaline earth oxides, have not been or are not readily transformed to transparent beads with significant microcrystallinity, and generally contain little or no alumina and zirconia.